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In logic, a type of inference or argument that purports to be valid, where a valid argument is one whose conclusion must be true if its premises are true (seevalidity). Deduction is thus distinguished from induction, where there is no such presumption. Valid deductive arguments may have false premises, as demonstrated by the example: All men are mortal; Cleopatra is a man; therefore, Cleopatra is mortal. Invalid deductive arguments sometimes embody formal fallacies (i.e., errors of reasoning based on the structure of the propositions in the argument); an example is affirming the consequent: If A then B; B; therefore, A (seefallacy; formal and informal).
This entry comes from Encyclopædia Britannica Concise. For the full entry on deduction, visit Britannica.com.